1. Field of the Invention
The present invention relates to a fully-differential amplifier, and more particularly relates to a fully-differential amplifier used for an output circuit of a photoelectric conversion apparatus.
2. Description of the Related Art
Due to an increasing demand for the resolution of solid-state image pickup devices, increasing the number of pixels that are provided in the solid-state image pickup devices is an important issue. Since the pixel size has been decreased to increase the number of pixels, the size of each of elements that are included in the solid-state image pickup devices has also been decreased. Further, the power voltage has been decreased to solve the problem of a decrease in the dielectric strength voltage of the element, the decrease being caused by the decreased element size, and an increase in the power consumption, the increase being caused by the increased number of pixels.
An interface to a signal-processing unit provided to process an output signal transmitted from a solid-state image-pickup apparatus, the interface being configured in a differential manner, has been disclosed in Japanese Patent Laid-Open No. 2008-067064. FIG. 8 is FIG. 3 of Japanese Patent Laid-Open No. 2008-067064. A fully-differential amplifier used in the solid-state image-pickup apparatus, which is disclosed in Japanese Patent Laid-Open No. 2008-067064, will be described with reference to FIG. 8.
According to a configuration illustrated in FIG. 8, when a differential signal is transmitted to an inverting input terminal Vin1 and a non-inverting input terminal Vin2, a differential current having a resistance value R1 as coefficient passes through resistances 64a and 64b. That is to say, a voltage input is converted into a current through the resistances 64a and 64b. 
Since the above-described differential current passes through resistances 65a and 65b, an output signal transmitted from the fully-differential amplifier is obtained as a voltage signal due to a voltage drop caused by the resistances 65a and 65b. That is, a current signal is converted into the voltage signal. The above-described fully-differential amplifier allows for determining a gain for an input signal based on the ratio between the resistance value R1 of the resistances 64a and 64b, and a resistance value R2 of the resistances 65a and 65b. Therefore, it is described that even though each of the resistance values R1 and R2 is changed due to manufacture variations, the amplification gain is not affected by the above-described change.
According to the configuration of the fully-differential amplifier illustrated in FIG. 8, the resistances 64a and 64b that are included in a voltage-current conversion unit 61 are in series with the individual resistances 65a and 65b that are included in a current-voltage conversion unit. Since a plurality of resistances is connected in series with respect to a path extending from a power voltage VDD to a ground voltage, a voltage drop occurring due to the above-described resistances limits the voltage range of output terminals Vout1 and Vout2. Therefore, it has been difficult to set the power voltage VDD to a low level while maintaining the signal amplitude and increase the amplitude of an output signal without increasing the power voltage VDD through the use of the configuration disclosed in Japanese Patent Laid-Open No. 2008-067064.